A casting assembly consists of a pouring cup, a gating system (including downsprues, choke, and runner), risers, sleeves, molds, cores, and other components. To produce a metal casting, metal is poured into the pouring cup of the casting assembly and passes through the gating system to the mold and/or core assembly where it cools and solidifies. The metal part is then removed by separating it from the core and/or mold assembly.
The molds and/or cores used in the casting assembly are made of sand or other foundry aggregate and a binder, often by the no-bake or cold-box process. The foundry aggregate is mixed with a chemical binder and typically cured in the presence of a liquid or vaporous catalyst after it is shaped. Typical aggregates used in making molds and/or cores are aggregates having high densities and high thermal conductivity such as are silica sand, olivine, quartz, zircon sand, and magnesium silicate sands. The amount of binder used for producing molds and/or cores from these aggregates on a commercial level is typically from 1.0 to 2.25 weight percent based upon the weight and type of the aggregate.
The density of a foundry mix is typically from 1.2 to 1.8 g/cc while the thermal conductivity of such aggregates typically ranges from 0.8 to 1.0 W/m.K. The resulting molds and/or cores are not exothermic since they do not liberate heat. Although molds and cores have insulating properties, they are not very effective as insulators. In fact, molds and cores typically absorb heat.
Risers or feeders are reservoirs which contain excess molten metal which is needed to compensate for contractions or voids of metal which occur during the casting process. Metal from the riser fills such voids in the casting when metal from the casting contracts. Thus the metal from the riser is allowed to remain in a liquid state for a longer period of time, thereby providing metal to the casting as it cools and solidifies. Sleeves are used to surround or encapsulate the riser and other parts of the casting assembly in order to keep the molten metal in the riser hot and maintain it in the liquid state. The temperature of the molten metal and the amount of time that the metal in the riser remains molten is a function of the sleeve composition and the thickness of the sleeve wall, among other factors.
In order to serve their function, sleeves must have exothermic and/or insulating properties. The exothermic and insulating thermal properties of the sleeve are different in kind and/or degree than the thermal properties of the mold assembly into which they are inserted. Predominately exothermic sleeves operate by liberating heat which satisfies some or all of the specific heat requirements of the riser and limits the temperature loss of the molten metal in the riser, thereby keeping the metal hotter and liquid longer. Insulating sleeves, on the other hand, maintain the molten metal in the riser by insulating it from the surrounding mold assembly.
Foundry molds and cores do not have the thermal properties which enable them to serve the functions of a sleeve. They are not exothermic, are not effective enough as insulators, and absorb too much heat to keep the molten metal hot and liquid. Compositions used in foundry molds and cores are not useful for making sleeves because they do not have the required thermal properties and density.
Typical materials used to make sleeves are aluminum, oxidizing agents, fibers, fillers and refractory materials, particularly alumina, aluminosilicate, and aluminosilicate in the form of hollow aluminosilicate spheres. The type and amount of materials in the sleeve mix depends upon the properties of the sleeves which are to be made. Typical densities of sleeve compositions range from 0.4 g/ml to 0.8 g/ml. The thermal conductivity for aluminum at room temperature is typically greater than 200 W/m.K while the thermal conductivity for hollow aluminosilicate microspheres at room temperature ranges from 0.05 W/m.K to 0.5 W/m.K. To some extent, all sleeves are required to have insulating properties, or combined insulating and exothermic properties in order to minimize heat loss and to maintain the metal in a liquid state for as long a time as possible.
Three basic processes are used for the production of sleeves, "ramming", "vacuuming", and "blowing or shooting". Ramming and blowing are basically methods of compacting a sleeve composition and binder into a sleeve shape. Ramming consists of packing a sleeve mix (sleeve composition and binder) into a sleeve pattern made of wood, plastic, and/or metal. Vacuuming consists of applying a vacuum to an aqueous slurry of a refractory and/or fibers and suctioning off excess water to form a sleeve. Typically, whether ramming, blowing, or vacuuming is used to form the sleeve, the sleeves formed are oven-dried to remove contained water and cure the sleeve. If the contained water is not removed, it may vaporize when it comes into contact with the hot metal and result in a safety hazard. In none of these processes is the shaped sleeve chemically cured with a liquid or vaporous catalyst.
These compositions are modified, in some cases, by the partial or complete replacement of the fibers with hollow aluminosilicate microspheres. See PCT publication WO 94/23865. This procedure makes it possible to vary the insulating properties of the sleeves and reduces or eliminates the use of fibers which can create health and safety problems to workers making the sleeves and using the sleeves in the casting process.
One of the problems with sleeves is that the external dimensions of the sleeves are not accurate. As a result, the external contour of the sleeves does not coincide in its dimensions with the internal cavity of the mold where the sleeve is to be inserted. In order to compensate for the poor dimensional accuracy, it is often necessary to oversize the cavity in the mold where the sleeve is to be inserted, or form or place "crush ribs" in the mold assembly which erode or deform when the sleeves are inserted into the riser cavity to provide a means of locking the sleeve in place. Alternatively, the sleeves are placed in position on the casting pattern and the mold is made around the sleeves, thus avoiding problems with sleeves that are not dimensionally accurate.
Another problem with sleeves is that they may lack the required thermal properties needed to maintain the molten metal in the riser reservoir in a hot and liquid state. The result is that the casting experiences shrinkage which results in casting defects. These casting defects are most likely to be scrapped which results in wasted time and metal.
Runners, sprues, and other components of the casting assembly also can use insulating and exothermic sleeves as coverings to maintain the temperature of the molten metal which comes into contact with them.